(1) Field of the Invention
The present invention relates to a method of marking or inscribing a workpiece with high-energy radiation, more particularly with a laser beam, the workpiece being transparent for the radiation wavelength, and a polymer matrix being disposed in the vicinity of the workpiece in such a way that the radiation passes through the workpiece before it impinges on the polymer matrix.
(2) Description of Related Art
It is known that the use of laser radiation for machining the material of a workpiece is based fundamentally on absorption of the radiation and energetic conversion with subsequent processes such as vaporization, ionization, particulate removal and photochemical events. These processes may take place in the workpiece itself, producing markings or inscriptions in the form, for example, of engraving, or else another material is deposited locally on the workpiece, as part of laser beam evaporation (pulsed laser deposition, PLD), for example. Variants of pulsed laser deposition take place in particular in the case of workpieces whose material is transparent for the wavelength of the laser radiation that is used.
The absorbing polymer matrix is disposed, for example, in direct contact with the transparent workpiece, and so the laser beam passes through the workpiece and, on impinging on the interface between workpiece and polymer matrix, vaporizes the material of the polymer matrix and is deposited on the surface of the workpiece in the form of a marking or inscription.
The methods known in this respect have a number of disadvantages. First, for maximum resolution of the marking or inscription, the distance between workpiece and polymer matrix must be as small as possible, so that the vaporized material is deposited over as small as possible an area of the workpiece. For this purpose, for example, a polymer matrix in the form of a transfer film may be adhered to the workpiece. If, however, the surface of the workpiece that is to be inscribed is rough, damp, uneven or soiled, there is often, at least locally, no direct contact between polymer matrix and workpiece. This may be caused by inclusion of air bubbles or other liquid or solid impurities or unevennesses. A distance between polymer matrix and workpiece then brings with it a number of disadvantages. First, there are two additional interfaces between the workpiece (optically dense) and air (optically thin), on the one hand, and air (optically thin) and polymer matrix (optically dense) on the other. This results in unwanted losses through scattering and reflection, with part of the power of the laser radiation not being available for the desired vaporization process, but instead, unwantedly, or even damagingly, heating up regions of the workpiece and/or of the polymer matrix. Secondly, the material removed from the polymer matrix is distributed over an areal region which is proportionately greater in line with the distance between polymer matrix and workpiece. As a result, the resolution of the marking or inscription becomes poorer. Moreover, the greater the distance between polymer matrix and workpiece, the smaller the fraction of the amount of material deposited on the workpiece as a proportion of the amount of material removed from the polymer matrix. Furthermore, in order to generate a particular reaction product which is to serve as a marking or inscription, it may be necessary for reactants provided in the polymer matrix to react in a closed reaction space to form the product. As a result of an opening to an air gap between polymer matrix and workpiece, the formation of such a reaction space may not be achieved. Finally, the removal of material from the polymer matrix produces what is referred to as smoke, which contains unwanted by-products which likewise originate from the polymer matrix. As little as possible of this smoke should become mixed on the workpiece in the form of debris in or around the marking or inscription. The production of smoke is greater when there is air included between polymer matrix and workpiece, and is a cause of untidy results.
In addition to a method with a solid polymer matrix, there are laser methods known which are based on increasing the near-surface absorption of laser radiation through a liquid. In this case, deliberately, an absorbing medium is used, typically an organic liquid. In this method, the transparent workpiece is in contact with the liquid at its back side, the side to be machined, and the laser radiation enters through the front side. The absorption of the laser radiation in the liquid produces, via heat transport from the heated liquid, a rapid increase in temperature to beyond the melting and vaporization point of the glass matrix. This method, which is referred to as laser-induced back side wet etching (LIBWE), leads to removal of material from the glass surface, and is used for the microstructuring or microengraving of glass.
Where a wetting absorber liquid is selected, it is indeed possible to avoid inclusion of air—unwanted for the reasons given above—between liquid and workpiece. A disadvantage of a liquid absorber, however, is that it does not allow localized removal of material and subsequent localized deposition on the workpiece, but instead only removal of material from the workpiece, produced by heat transfer—in other words, for example, only engraving.
It is an object of the present invention, therefore, to provide an improved method of marking or inscribing a workpiece with high-energy radiation, more particularly with a laser beam, in which the disadvantages of the known methods are overcome and it is possible to carry out marking or inscription even of rough, damp, uneven or soiled surfaces of the workpiece with high quality and resolution.
This object is achieved by a method according to Claim 1. Preferred embodiments are subject matter of the dependent claims.